Process for tanning hides

Abstract

A process for tanning hides for soles or insoles with ammonium titanyl sulphate. The hides, subjected to soaking, liming, fleshing, and deliming, are first treated with ammonium sulphate. Successively, ammonium titanyl sulphate as well as citric acid or an alkali metal citrate are gradually added to the bath, while maintaining the temperature of the bath at a value not exceeding 37° C. and a pH at values ranging from 1.7 to 2.5. After each addition, the hides are treated until they have absorbed at least 90% of the soluble Ti (IV). Sodium sulphite and hexamethylenetetramine are then gradually added: this addition is such as to bring the final pH of the bath to 3.8-4.5 and to exhaust the bath in soluble Ti (IV). NaHCO 3 is successively added until a pH value of the bath ranging from 4.0 to 5.0 is reached, such pH being maintained for at least 2 hours. At least one tanning treatment with a tannin, either prior to the treatment with ammonium titanyl sulphate (pretanning), or after the treatment with NaHCO 3 (additional tanning), is carried out, said tanning being accomplished with a synthetic or vegetable tannin in the former case, and with a synthetic tannin in the latter case.

What is claimed is: 
     
       1. A process for tanning hides for soles or insoles in which the hides, after having undergone soaking, liming, fleshing, and deliming, are tanned with ammonium titanyl sulphate, characterized in that the hides are first treated with ammonium sulphate; successively ammonium titanyl sulphate and citric acid or an alkali metal citrate are gradually added to the bath, maintaining the bath temperature at a value not exceeding 37° C. and the bath pH at values ranging from 1.7 to 2.5; after each addition the hides are treated until they absorb at least 90% of the soluble Ti (IV); a gradual addition of sodium sulphite and hexmethylenetetramine is then carried out, this addition being such as to bring the final pH of the bath to 3.8-4.5 and to exhaust the soluble Ti (IV) in the bath; NaHCO 3  is then added till reaching a pH of the bath ranging from 4.0 to 5.0, such pH being maintained for at least 2 hours; at least one tanning treatment with a tannin, either before the treatment with ammonium titanyl sulphate (pretanning), or after the treatment with NaHCO 3  (additional tanning) is carried out, said tanning being accomplished with a synthetic or a vegetable tannin in the former case, and with a synthetic tannin in the latter case. 
     
     
       2. The process according to claim 1, characterized in that in the tanning step with ammonium titanyl sulphate, from 15 to 50% by weight of citric acid or alkali metal citrate (calculated as citric acid) with respect to ammonium titanyl sulphate (calculated as TiO 2 ) is used. 
     
     
       3. The process according to claim 1 or 2, characterized in that the pretanning is accomplished as follows: the hide pH is brought to values ranging from 3.8 to 4.5 by treatment with a slightly tanning acid or with a non-tanning acid; successively the hides are treated with a synthetic or a vegetable tannin. 
     
     
       4. The process according to claim 1 or 2, characterized in that the additional tanning is carried out with a synthetic tannin or with a synthetic tannin and a rubber-like polymer. 
     
     
       5. The process according to claim 1 or 2, characterized in that when a pretanning has taken place, the additional tanning is carried out with a rubber-like polymer. 
     
     
       6. The process according to claim 3, characterized in that the slightly tanning acid is selected from the group consisting of α-naphthalenesulfonic acid, β-naphthalenesulfonic acid, and mixtures thereof. 
     
     
       7. The process according to claim 3, characterized in that the non-tanning acid is selected from the group consisting of formic acid, acetic acid, and sulphuric acid. 
     
     
       8. The process according to claim 1 or 2, characterized in that the synthetic tannin is selected from the group consisting of a phenol and formaldehyde polycondensate, a naphthol and phenol and formaldehyde polycondensate, and a naphthalenesulfonic acid polycondensate. 
     
     
       9. The process according to claim 4, characterized in that the rubber-like polymer is selected from the group consisting of a polychloroisoprene, a butadiene-styrene copolymer, a silicone, a polymethylacrylate, a polyethylacrylate, and a polymethylmethacrylate. 
     
     
       10. Leather for soles or insoles when tanned by the process as claimed in claim 1.